When printing photographs or masters having individual areas with large differences in brightness, the prints are frequently overexposed in the light regions and underexposed in the dark regions. As a result, details and fine structures are not, or are only poorly, reproduced on the prints.
The German Offenlegungsschrift 31 41 263 describes a method of copying color diapositives on reversal paper using masks for contrast reduction. The diapositive is placed in direct contact with a phototropic sheet of glass and ultraviolet radiation or the like is then passed through the diapositive into the phototropic sheet. A black-and-white negative mask of the diapositive is thus produced in such sheet. With the mask and the diapositive in the same relative position, radiation is thereupon passed through the composite of mask and diapositive in the opposite direction. In this manner, the diapositive is printed on the reversal paper with low contrast.
It has now been found that phototropic glass is not color neutral. This means that darkening of the glass causes different spectral portions of the printing light to undergo different reductions in intensity.
Another photographic printing apparatus designed for masking is disclosed in U.S. Pat. No. 4,239,385. According to this patent, the mask is generated on an LCD matrix which can be regulated by an electronic control unit and is disposed between the illuminating system and a photograph to be printed.
Here, again, there is the problem of color shifting upon darkening of the LCD matrix. A further problem in the apparatus of the U.S. patent stems from the temperature-dependence of LCD displays. Since the degree of darkening of the matrix varies with temperature, reproducible control is very difficult to achieve.